Strain gauge center of gravity and gross weight meter



June 8, 1948.

P. M. MAGRUDER ET AL STRAIN GAUGE CENTER OF GRAVITY AND GROSS WEIGHT METER 2 Sheets-Sheet 1 Filed March 9. 1944 UKDOE INVENTOR.

WELCOME w BENDER JR.

UDER

PEY M. MA BY av 4n7 gz TORNEIY June 8, 1948.

STRAIN Filed March 9, 1944 FIGURE 3.

P.'.M. MAGRUDER ET AL 2,443,045

GAUGE CENTER OF GRAVITY AND GROSS WEIGHT METER 2 Sheets-Sheet 2 FIG LJRE 4 FIGURE 5.

INVENTOR.

WELCOME VV. .BENDER JR.

PEYTON M. MAGRUD ER AT To Y:

Patented June 8, 1948 STRAIN GAUGE CENTER OF GRAVITY AND GROSS WEIGHT METER Peyton M. Magruder, Baltimore, and Welcome W. Bender, Jr., Towson, Md., assignors to The Glen L. Martin Company, Middle River, Md., a corporation of Maryland Application March 9, 1944, Serial No. 525,652

4 Claims. 1

whereby the center of gravity and gross weight can readily be determined without the usual expedients employed and a ground crew of several men.

By this invention a Self-contained instrument is provided whereby gross weight and center of gravity can be determined prior to flight.

A further object of this invention is the provision of an instrument that can readily be adapted to almost any airplane with no structural change.

Further and other objects will become apparent from the description of the accompanying drawings which form a part of this disclosure and in which like numerals refer to like parts.

In the drawings:

Figure 1 shows a front elevation of an airplane with invention attached.

Figure 2 is a side elevation explanatory of this invention.

Figure 3 shows asingle wheel landing gear strut with the invention attached. 7

Figure 4 shows a double wheel landing gear with the strut attached.

Figure 5 shows the circuit diagram of the invention.

Figures 1 and 2 oi. the drawing illustrate a conventional airplane having a tricycle landing gear system. Nose wheel strut I has secured thereto strain gauges 2 and 3 on that portion of the strut 4 bearing a direct load of the airplane. Main landing gear struts 5 have strain gauges 6 and I mounted in a similar manner. The signals produced by these electric strain gauges mounted on the landing gear struts in a particular fashion actuates a meter or indicating device whose scale is calibrated in terms of gross weight and center of gravity location. The strain gauges employed are preferably of the metalectric type which are of bonded resistance wire and are mounted, cemented or otherwise suitably attached to the part of the landing gear strut in such a. way that their resistance varies in proportion to the vertical load supported by each structure. In the case of the conventional tail wheel type airplane gauges can be attached either to the tail wheel mechanism or some part of the aft fuselage structure whose stress varies substantially in comparison to the vertical load on the tail support mechanism,

Figure 3 shows an enlarged view of the nose wheel and strut which is also typical of the main landing gear. Strain gauges 2 and 3 are attached to the portion 4 which is subject to a bending stress due to vertical loading on the wheel 21. Gauge 3 is mounted on the tension side and gauge 2 is mounted on the compression side of the strut. These gauges are wired in adjacent arms AC and CB respectively of a Wheatstone bridge circuit 1 as shown in Figure 5 so that the bridge balance is changed by diilerential strains in the strut due to bending, but is unaifected by the uniform strains due to axially loading or moreespecially due to temperature changes. Arm AC of the bridge contains the tension gauges 20 on both main gear struts wired in series. While arm BC contains the compression gauges 2| on both struts wired in series. 7

In the case of a vertical strut not subject to bending, as for instance a vertical strut on a large airplane supported by two wheels 21 as shown in Figure 4. temperature compensation is accomplished by mounting one gauge 8 parallel to the principal stress and a gauge 9 perpendicular to it as shown in Figure 4. Gauge 8 is the compression gauge while gauge 9 is the tension gauge. By reason of Poissons ratio of strain in an elastic body these gauges will react difierently to axial loads on the strut and thereby change the balance of the bridge but they willreact uniformly due to strains produced by temperature changes and will thereby not effect the bridge balance.

By the arrangement of the gauges described above for the two types 01' struts, temperature effects which might otherwise seriously discount the usefulness of this device by impairing its accuracy are reduced to a negligible quantit in that the arrangement of the gauges in the bridge renders it compensating for temperature changes.

With e ther arrangement of gauges, the gauges from both main gear struts are wired in series in arm AC and BCof the bridge as shown in Figure 5, in order to obtain a bridge output proportional to the sum of the loads on the main gears regardless of the lateral asymmetry of the airplane load. Gauges are also mounted on the nose gear or tail gear as the case may be and are similarly compensated for temperature and connected into arms AD and BD of the bridge circuit. Tension gauge 22 is wired in arm BD and-compression gauge 21 in arm AD. It is evident from bridge circuit theory that a small variation of resistance in arm AC is equivalent to a similar small variation in arm BB in that both produce similar unbalanced voltages across the bridge, and their effect is additive. Therefore, the unbalanced voltage appearing across CD is proportional to the sum of the effect produced by gauges 20 on the main gear and gauges 22 on the nose gear or the unbalanced voltage is equal to (M+N) The bridge circuit ABCD shown in Figure is provided with suitable balance resistors II and otentiometer il which is adjustable to balance the bridge in some fixed or reference loading of the strain gauges, as for instance during flight where there is no load on the main landing gear, or on the ground when the ship is jacked u and the landing gear is free of load.

The airplane gross weight is equal to the sum of the weights supported by the main gear struts and the nose gear strut, but since the geometrical proportions of the nose gear are in general not identical with those of the main gear struts, it it not likely that strain gauges mounted on the nose gear strut will have the same rate of change of resistance with vertical load as the gauges mounted on the main gear struts. Therefore, if the bridge signal response is to be made proportional to the sum of the three vertical loads, means must be provided for adjusting the sensitivities of the various gauges to some uniform value in terms of the vertical load being supported by the respective struts.

For the purpose just stated, a potentiometer I2 is provided for adjusting the sensitivity of the nose strut gauges on the bridge output, measured across points C and E when the switch II is in the "up" or gross weight position. Potentiometer i2 is adjusted during initial installation and calibration of the device. It is set at a position which will produce the same deflection of the meter, for a 1000 pound up load on the nose gear, as is produced by a 1000 pound "up load on the main gear. In the event of any appreciable manufacturing dissymmetry 0f the two main gear struts, similar potentiometers can be installed across legs AC and AB to allow balance of the sensitivity of both main gear struts, though this is hardly likely to be necessary.

The scale of meter I6 is calibrated in terms of gross weight of the airplane, and for practical cases need only cover the range between weight empty and maximum gross weight of the airplane.

Since the sensitivity of any bridge circuit is proportional to the applied voltage across AB, this voltage must be held substantially constant if the meter calibration in gross weight is to be read directly. Therefore, a suitable neon glow lamp ll or other voltage regulator is wired into the circuit to maintain the potential across AB even though the voltage of the electric power source It, of Figure 5, varies in the normal operation of the airplane. A potentiometer 24 is included for coarse adjustment of battery voltage. A further suitable resistor 25 is included together with switch 26 to provide a means for checking the bridge voltage on the meter IS, the circuit of which contains fuse 29. The adjustment of the bridge voltage is accomplished by variable resistors.

The method of determining the center 01' gravity position can best be explained by refer- 4 ence to Figure 2 before studying further ieatures of the circuit of Figure 5. As seen in the Figure 2, the gross weight is again the sum of forces M and N, M representing the vertical force on the main gear, and N the vertical force on the nose ear.

The fore and aft position of the center of gravity can be deiined as the ratio A/L, since L is a iixed constant length dependent upon the airplanos geometry. Now, since for equilibrium of the airplane. the sum of the moments about the center of gravity is zero,

MANB=0 (1) now substituting B=L-A in this equation and transposing, we obtain MA=N(LA) =NL-NA (2) transposing again we obtain MA-l-NA-NL=A(M+N) (3) Now solving Equation 3 for A/L we obtain finally A/L=N/(M+N) (4) Thus it has been shown that the center of gravity is directl proportional to the load on the nose gear and inversely proportional to the gross weight.

With these relations in mind, the center of gravity features of the circuit of Figure 5 have been designed. With the double pole, double throw switch It in the down or "center of gravity" position, the meter reading is proportional to the load on the nose gear. Potentiometer I1 is adjusted with no load on the nose gear strut during initial balancing and calibration of the secondary bridge ABDG, which contains the arms 20 and the gauge arms AD and BD.

In order to correct the meter reading for the efi'ect of gross weight in Equation 4, the variable resistor ll of a potentiometer type is included in the circuit. This variable resistor adjusts the resistance in the meter circuit which results in a proportional variation of current in the meter circuit produced by the unbalanced voltage across DG. In other words it adjusts the meter sensitivity to compensate for gross weight when measuring center of gravity. A dial II is provided for adjusting potentiometer ll as shown, and the scale of this dial is calibrated in terms of gross weight.

Now in order to measure the center of gravity, after the gross weight has been determined as described above, the pointer of potentiometer ll is set to this gross weight on the dial is of potentiometer II and the switch I! is thrown to the center of gravity position. The center of gravity position is then read from the meter I on a suitably designated auxiliary scale. This scale is calibrated in terms of A/L or as is customary in defining the center of gravity, in terms of percent of the mean aerodynamic wing chord, or in any other convenient units.

It is further pointed out in the operation of the potentiometer II that the meter circuit resistance varies approximately with the gross weight setting of the dial of potentiometer ll. Thus for large gross weights, the meter circuit resistance is large, and its sensitivity small, while for small gross weights the opposite is true. Thus the effect of gr s weight is compensated by potentiometer II and the center of gravity is directly proportional to the meter reading which is in turn proportional to the nose gear load.

In considering the practicability of this invention, it must be pointed out that the strain gauges must be designed to. carry the current required and yet have suil'iciently high and constant sensitivity over a reasonably long period of time to make the device useful in airplane operation.

It has been ointed out above that this device can be used with only slight modifications to measure lateral position oi the center of gravity if it becomes important or useful to do so, as for instance in an unsymmetrically loaded freight carrying airplane.

The term meter" used in this specification means not only the conventional moving pointer indicator but also any form of measuring device such as a bridge balancing potentiometer or sensitive galvanometer.

It is to be understood that certain changes, alterations, modifications and substitutions can be made without departing from the spirit and scope or the appended claims.

We claim as our invention:

1. In an airplane having strut-supported tricycle landing gear, metalectric strain gauges mounted on said struts and so arranged on each strut that one gauge measures the load and one gauge compensates for temperature on that strut, said strain gauges being secured to a, portion oi the strut which is subject to strain in supporting its share of the weight of the airplane and load. an electric bridge circuit generally of the Wheatstone type including said strain gauges and a meter. the main landing gear strain gauges connected in two adjacent arms of said bridge with similarly stressed gauges connected in series in each arm, gauges on the third landing gear strut connected in bridge arms with similarly stressed connected to give an indication proportional to gross weight.

2. In an airplane having strut-supported tricycle landing gear, metalectric strain gauges mounted on said landing gear assembly with one gauge arranged parallel to the principal stress in the landing gear ior measuring load and one gauge arranged on the landing gear for temperature compensation, an electric bridge circuit genemlly oi the Wheatstone type including said cycle landing gear, metalectric strain gauges mounted on said struts and arranged, one parallel with, and one transversely of the principal stress to compensate for variations in temperature, said strain gauges being secured to a portion oi the strut which is subject to strain in supporting its share of the weight of the airplane and load, an electric bridge circuit generally of the Wheatstone type including a meter to which said strain gauges are connected, the main landing gear strain gauges connected in two adjacent arms of said bridge with similarly stressed gauges connectedin series in each arm, gauges on the third landing gear connected in opposite bridge arms, said strain gauges being so connected that the bridge output voltage is proportional to the sum 01 the strain gauge changes in resistance which will give an indication of the gross weight on the meter.

4. In an airplane having strut-supported tricycle landing gear oi' the type having the third gear on the opposite side of the center 01' gravity from the main landing gear, metalectric strain gauges secured to each strut with one gauge args ranged for measuring load and one gauge arranged for temperature compensation, said strain gauges being secured to a portion of the strut which is subject to strain in supporting its share of the weight of the airplane and load, an electric 80 bridge circuit of the Wheatstone type, including said strain gauges and a meter, said bridge having said strain gauges of the third gear connected in said bridge circuit in adjacent arms of said bridge, said meter connected to the junction between said gauges and the junction between the opposite arms 01' said bridge to indicate unbalance of the bridge due to the load on said third gear, a variable resistance means in said meter circuit to introduce a predetermined amount 01' resist- 40 ance proportional to the gross weight of the airstrain gauges and a meter. the main landing gear strain gauges connected in two adjacent arms of said bridge with similarly stressed 80-118 8 connected. in series in each arm, gauges on the third landing gear connected in bridge arms with similarly stressed gauges in arms opposite those of plane to adjust the sensitivity of the meter circuit for a known gross weight to give an indication of center oi gravity on said meter.

" PEYTON M. MAGRUDER.

WELCOME W. BENDER, Jn.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,407,078 Murray Feb. 21, 1922 1,864,878 westrum June 28, 1932 2,316,975 Ruge Apr. 20, 1943 2,373,504 Bchlieban et al. Apr. 10, 1945 FOREIGN PATENTS Number Country Date 387,887 Great Britain Feb. 16, 1033 OTHER REFERENCES "Characteristics and Aircraft Applications of Wire Resistance Strain Gages," by A. V. dc Forest. Instruments, vol. 15, April 1942, pages 112, 113, 114, 136 and 137.

Technical Notes, National Advisory Committee for Aeronautics, No. 744. "The Development of Electrical Strain Gages," by A. V. de Forest and H. Leaderman, published Jan. 1940. Page 27 is particularly relied upon as a reference. 

